Method of scaling stainless steel



Nov. 27, 1945. A. w. BROMBERG 2,389,838 7 METHOD OF SCALING STAINLESS STEEL Filed May 1, 1942 INVENTOR ///3r 4770/ 0 ey.

. in a minimum time in. a continuous atented Nov. 27, 1945 smas 'ATENT OFFICE This invention relates to improvements in the scaling of stainless steel for the removal of slivers and other surface defects therefrom.

In the production of stainless steels, the hot rolled metal contains surface defects, the more prominent of which are known as slivers, which are surface discontinuities formed during rolling. Such defects are difficult to remove, and since their elimination is essential to the perfection of the ultimate cold rolled product, the problem of such removal is of great importance to the industry.

Prior practice has included annealing in an oxidizing atmosphere and acid pickling of hot rolled stainless steel for removal of such defects, but as formerly practiced, the said treatment has been found to involve considerable expense. In this connection, particular reference is made to the production of chrome-nickel stainless strip, which is passed through an annealing furnace and one or more pickling baths in hot rolled form, as a continuous operation for annealing and for sliver removal. Although such treatment lines are of considerable length, I have found that too frequently the strip does not become properly deslivered or cleaned and must be reconditioned in order toremove sufficient metal to eliminate the more prominent slivers and defects.

Such re-treatment is costly, consuming extra time during which the equipment is off production schedule, and involving re-coiling and rehandling of the strip which interferes with continuous mill production.

An object of my invention is to promote the formation of a deep but easilyremovable scale on such hot rolled stainless steels so as .to remove sufiicient metal, including the defects and slivers, process of annealing and pickling of such steel.

I have discovered'that the application of a I scale accelerator to the steel prior to and in conjunction with annealing in a reducing atmosphere will greatly enhance the formation of scale and the removal of the defect-containing-scaled metal,

, including a loosening and partial removal in the annealing furnace as well as following the anneal-' mg, and a. complete final removal in the pickling bath.

Additional objects and advantages will become apparent from the following description taken in connection with. the accompanying drawing.

In the drawing, which diagrammatically illustrates a continuous annealing and pickling line embodying my invention, the hot rolled chromenickel stainless steel strip 2 is fed from a coil by feed rolls 3 through an annealing furnace 4i, following which the strip is quenched by water sprays 5, and thence passed successively through pickling baths or tanks 6, l and 8. Suitable means (not shown) are employed in the conventional manner for pulling the strip through the apparatus, and for coiling the strip after completion of the treatment.

My invention includes the application of a scale accelerator to the strip prior to annealing, as by means of a tank 9 having suitable rolls associated therewith for submerging the strip in a liquid solution of the accelerator. The purpose of such step in the method is totreat the strip with-a substance in the nature of a fluxing agent which will promote a substantially deep scale formation when the coated metal is annealed in a reducing atmosphere, enabling the removal of sufficient metal from the surface of the strip to also remove the undesirable defects therefrom.

I have discovered that when the strip is coated I with an aqueous solution of halide salts and subjected to annealing heat in a smoky or reducing atmosphere, the desired rapid scale formation is produced. The-solution first dries on the strip as the same enters the furnace, and then melts into a glossy coating. As the strip approaches the exit end of the furnace, the coating blisters and partially flakes off, and upon being quenched by the water sprays 5, large quantities of the formed scale are removed before pickling. The annealing temperature in said furnace may be from about 1800 to 2250 F. for said chromenickel stainless steels. i

The strip then passes into and through the pickling baths 6, 1 and 8, the first two of which may contain heated hydrochloric acid or sulphuric acid and sodium chloride of a strength of from about 15%.to 25% by volume. The final bath or tank 8 preferably contains a mixture of nitric and hydrofluoric acids, following which,

the strip is washed, dried, wiped and coiled the usual manner.

Comparison of the finished product withthat annealed in an oxidizing atmosphere and pickled to the same degree of pickling, shows a greatly The use of my invention has reduced the cost of the cleaning treatment of hot rolled chromenickel stainless steel strip as much as 40%.

The preferred scaling accelerator solution employed in the practice of my invention consists.

assasss erably being heated to a sufllcient degree for admixture. heated to from about 100 to 200 F. in the tank 7 9 positioned immediately in advance of the furnace 4. The following proportions of the ingredients of the solution have been found suitable, viz., sodium fluoride 1.3 sodium chloride 8.3%, and water 90.4% by weight.

Increasing the sodium chloride concentration was found toincrease the promotion of scale,- and solutions with as much as 15.0% of sodium chloride wi 1.5% of sodium fluoridescaled the strip efl'ectively. Varying amountsof t e salts of the mixture may be employed up to the sodium chloride-sodium fluoride eutectic mixture (34.5 molar percent sodium fluoride) beyond which the mixture becomes less'eflective for the promotiouof scale. It will therefore be seen that I have provided'a.

method of scaling and cleaning stainless steel which is of material advantage in the production of the finished metal, whereby the strip receives a uniformly cleansliver-free surface, which, to an For example, the solution -may beappreciable degree, insures an excellent uniform surface upon the flnished cold rolled product. It

is likewise apparent that the present method materially reduces the cost of production of stainless steel strip over present practices.

Various changes and modifications will become apparent from the scope of the following claims.

I claim:

1. The steps in the method of producing stainless steel strip which consist in coating thehot rolled strip prior to annealing with anaqueous solution, containing sodium chloride and sodium fluoride in varying amounts up to the eutectic mixture thereof, and then annealing the coated strip in a reducing atmosphere, whereby to promote increased scale formation of a character readily removable by acid pickling and in-an amount sufiicient to effect substantial removal of surface defects from the strip.

2. The herein described process of scaling chroma-nickel stainless steels which consists in coating the hot rolled strip prior to annealing with an aqueous solution of sodium fluoride and sodium chloride, annealing the coated strip ina reducing atmosphere at'a temperature of from about 1800 to 2250 F; to promote the formation of a scale thereon in substantial amount. quenching the scaled strip to remove some of the scale,

and then pickling the quenched strip and remaining scale in an acid bath, the said scale formation and removal/substantially removing surface defects from the strip.

ALFRED W. BROMIBERG; 

